1. Field of the Invention
The present invention relates to vibration dampers and, more particularly, to adjustable hydraulic vibration dampers.
The hydraulic adjustable vibration dampers have a damping piston fastened to a piston rod, which divides the work cylinder into two chamber halves filled with damping fluid. A bypass, formed by a transverse boring located above the damping piston and a longitudinal boring at least in the termal region of the piston rod, connects the upper half of the chamber with the lower half of the chamber. There is an axially-movable adjustment apparatus to adjust the damping force connected with an electromagnet, consisting of a coil and armature, to control the bypass.
2. Description of the Prior Art
Federal Republic of Germany Laid Open Patent Application No. DE-OS No. 33 03 293 describes damping force regulation devices for telescoping shock absorbers on which there is an additional double-acting damping valve, which is incorporated into the damping piston itself. These damping valves are connected in series and can be separated from one another by means of a rotary disc valve. In a first valve position, only the valve of the damping piston is carrying a flow, so that a mild damping action can be achieved. In a second valve position, the valve in the damping piston and the additional valve both carry flows, so that a correspondingly hard damping characteristic can be achieved.
A disadvantage with such an embodiment is that the rotary disc valve must be pressurized by appropriate adjustment forces, which can only be applied by slow-acting drives, such as pneumatically or hydraulically driven positioning mechanisms, or an electrically-driven positioning motor.
In Federal Republic of Germany Patent No. DE-PS 29 11 768, there are described adjustable shock absorbers in which there is a coil in the upper part of the piston rod and in which an armature can move axially. As a function of the current fed to the coil, a specified bypass opening can be created between the upper and lower work chamber. A disadvantage with this arrangement is that, by controlling a change in the cross section, it is not possible to achieve an optimal damping force characteristic. The cross section modification which can be achieved corresponds to the regularities of a simple throttle hole, whereby with increasing piston velocities, an undesirable progressive damping characteristic always takes over. Both of the above-mentioned Federal Republic of Germany Patent Publications are incorporated herein by reference as if they were set forth herein in their entirety.